Studies of catechol-O-methyltransferase (COMT), an enzyme important in the extraneuronal inactivation of catecholamines (dopamine, norepinephrine, and epinephrine) and the detoxification of xenobiotic catechols, will be undertaken in an effort to obtain the information necessary for the rational design of COMT inhibitors. By studying the metabolism of 3,4-dimethoxy-5-hydroxybenzoic acid, and its inhibitory effects on O-methylation in vivo, we should improve our understanding of how conjugative metabolism (sulfation and glucuronidation) affects the duration of action and effectiveness of COMT inhibitors. By studying the interaction of affinity labeling reagents with highly purified COMT in vitro, it should be possible to identify the crucial nucleophilic residues at the active site of COMT as well as the enzyme's mechanism of catalysis. Such information should lead to the eventual development of more effective and specific inhibitors of COMT as potential chemotherapeutic agents.